Deep vein thrombosis (DVT) is a common and significant complication of surgeries that require anesthetizing the patient. Studies have shown that the risk for DVT occurrence in orthopedic patients having lower extremity surgeries such as total knee arthroplasty, total hip arthroplasty, and hip fracture repair, may be as high as seventy percent. The clinical manifestations of DVT can include tenderness, edema, and discoloration; however, these physical findings typically are present in only twenty-three to fifty percent of the patients with DVT so there often is no visible warning of DVT complications. Post-thrombotic syndrome from DVT may arise in five to ten percent of DVT patients with symptoms presenting in the form of chronic edema and ulcers of the lower extremities.
The most significant complication arising from DVT is pulmonary embolism. Pulmonary emboli are recognized as the most common fatal complication following surgery or trauma involving the lower extremities. Pulmonary emboli (PE) are the principal or major contributing factor in the cause of death in approximately fifteen to thirty percent of all hospitalized patients and result in an estimated 50,000 to 100,000 deaths per year. The rates of clinically significant pulmonary embolism in orthopedic patients have been reported to be as high as twenty percent with a one to three percent incidence of fatality. This is significant as otherwise successful surgical procedures can result in a loss of life, often with little or no warning.
The generally accepted theory on the formation of thrombi includes vessel wall damage, hyper-coagulability, and stasis. It is believed that stasis can lead to the development of vessel wall damage and hyper-coagulability.
Deep vein thrombi predominantly form in the deep veins of the legs which are surrounded by the gastrocnemius and the soleus muscle groups. DVTs initially form distally in the deep veins and grow proximally prior to their potential release as emboli. Contraction of the gastrocnemius and the soleus muscle groups activate the body's "skeletal muscle pump" by compressing the deep veins and mechanically pushing blood back toward the heart. The blood flow is unidirectional as veins contain valves which permit the flow of blood in only one direction (towards the heart). The action of the "skeletal muscle pump" is a process which occurs nearly continuously in ambulatory people. Even while sleeping, a person's body periodically shifts and muscles twitch which activate the "skeletal muscle pump."
During surgery, the body's "skeletal muscle pump" can be compromised due to the effects of general anesthesia and paralytic agents administered to facilitate, for example, intubation. This compromise of the "skeletal muscle pump" can lead to stasis of blood in the deep veins, formation of deep vein thrombi, and ultimately the formation of emboli. This situation is further aggravated as surgical patients tend to be non-ambulatory for a significant period of time after surgery. Without ambulation, the "skeletal muscle pump" is only marginally effective. Accordingly, prevention of the blood stasis in the deep veins during surgery and subsequent augmentation of blood flow in the post-surgical period represents a very important window of opportunity for preventing the formation of DVTs.
Many techniques have been proposed and utilized in an attempt to prevent the formation of thrombi. In particular, pharmacological and mechanical modalities have been studied.
Pharmacological efforts have been primarily targeted at blood hypercoagulability using either anti-platelet drugs or anti-coagulant blood thinners such as heparin and coumadin. These pharmacological techniques have been at least partially successful in preventing DVTs. Studies have shown that DVT rates can be decreased by over fifty percent depending on the specific drug utilized, surgery, and administration protocol. Such drugs are effective because they cause blood thinning and prevent coagulation. Therefore, physicians are not particularly enthusiastic about using these drugs as they can cause bleeding complications, they are expensive, they are logistically complicated to use, and the general trend is moving away from the use of such drugs.
Alternatively, stasis can be prevented by the use of mechanical devices such as Sequential Compression Devices (SCDs) that compress or squeeze a patient's legs to force blood flow towards the heart, foot pumps that rapidly compress the plantar tendon to push blood from the arch area, or compression stockings that provide support to blood vessel walls. These devices have met with success and have helped to decrease DVT rates by approximately sixty to eighty percent.
Commercially available devices, however, are fraught with drawbacks. They typically are bulky and impose a considerable burden on the hospital staff. Patients are less inclined to ambulate while using these devices because they typically are burdensome to remove and reapply. Early and frequent ambulation following surgery is strongly correlated with quicker overall recovery and decreased risk of DVT/PE. Conventional devices are expensive and difficult to use during surgery. Moreover, since conventional devices are durable goods owned and maintained by the hospital, they are not suited for post-operative home care, during which studies have shown that active DVT prophylaxis is still necessary. Additionally, patient compliance is typically very poor when using the mechanical devices as they are uncomfortable, interfere with sleep and may cause accumulation and stagnation of perspiration, resulting in unpleasant odors.
Electrical stimulation of muscle groups of the lower extremities has been shown to be effective for preventing DVT. A number of electrical stimulation devices exist and have been used to cause muscle contractions. Included in this class of electrical stimulating devices are the transcutaneous electrical nerve stimulators commonly known as TENS units. TENS units have been primarily utilized in post-surgical or non-surgical situations for the reduction or minimization of pain. The TENS units are designed to block pain at the level of the nerve endings.
A more recent development in the use of electrical stimulation to prevent DVT has been the use of neuro-muscular electrical stimulation (NMES) on the lower extremities to promote venous return of blood as disclosed in U.S. Pat. No. 5,556,422, issued Sep. 17, 1996. The NMES device uses tetanic frequency stimulation to cause dorsiflexion neutral inversion/eversion muscle contraction which is utilized for promoting venous blood return from the deep veins in a patient's leg. The dorsiflexion and stretching stimulated by the repeated application of an electrical current at a predetermined, regular interval has been found to increase the blood flow in deep leg veins. This type of electrical stimulation causes significant, sustained muscle contraction and movement about the ankle joint and, therefore, is not useful during surgery. The substantial movement caused by the dorsiflexion may be adverse in a surgical situation wherein a particular surgical procedure requires the patient to be virtually immobile during the procedure. An additional drawback of this type of stimulation is that it can be painful and fatiguing to the patient.
Accordingly, it would be desirable to have a device and method that prevents DVT, which can be utilize in the intra-operative setting, the post-operative setting, and/or therapeutic settings. This invention meets that need and overcomes the disadvantages of the prior art devices described above.